# rc filter testing, oscilloscope and function generator

Discussion in 'Homework Help' started by ninjaman, Mar 9, 2014.

1. ### ninjaman Thread Starter Member

May 18, 2013
306
1
i have to make an RC filter, resistor in series and capacitor in parallel. the values are 1k and 100nF.
I have to use a function generator and oscilloscope to find the critical frequency which I believe is the same as cutoff frequency(the 0.707 bit)
I have to draw the frequency response curve.
I think I know how to do this. my lecturer is off sick so I haven't been able to get any help though I have found some things on the internet.

so far, I make the circuit. connect the function generator across the input. then channel 1 of osc to input, channel 2 of osc to output and connect the ground to ground of circuit.
this way I have an input waveform and an output waveform. I think the voltage remains the same at 1volt. I think I have to increase the frequency and measure the output voltage. I should be able to see the difference in voltage between the two wave forms?

im not sure about this. I emailed my lecturer but I think he may be a little too ill as I have no answer.
after this I have to construct an op amp circuit. input resistor is to negative and 10k, the feedback resistor is 100k. it has two 15v supplies.

I have to, measure and record the gain and bandwidth of the amp. draw the frequency curve.
change the value of the feedback resistor to give a gain of -100

measure and record the gain and bandwidth of the amp, draw frequency curve and comment on results of both.

compare the results of both virtual and practical aspects of this assignment in terms of ease or measurement, errors and accuracy of results.

I will be using excel to show a response curve using graph.

I think I may be correct in the testing of the filter, the op amp im not to sure about. I don't know how to measure the bandwidth. I don't know how to work a function generator that well either. it only goes to about 1v. we have power supplies at college.

any help would really be appreciated!!!!
thanks
simon

2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
511
resistor in series and capacitor in parallel-this part does not make sense to me.

RC filter could be high pass filter or could be low pass filter. Which one are you doing? Are you doing both?

The job of the filter is to pass through certain, user determine, frequencies and block all other frequencies.

If you use voltage divider style RC circuit, this type of RC filter has a cutoff frequency of Fc=1/(2*pi*RC). Since your R is 1 kOhm, your C is 100 nF, your Fc is 1591.5 Hz, round it up to 1.6 kHz.

Now you hookup the o-scope.

When you hookup o-scope across the capacitor, you will have low pass filter. You will be putting in 1 volt at frequencies 0 to lets say 1.5 kHz to the RC circuit. You will see 1 volt across the capacitor. Then you do the same for several frequencies that are greater than 1.6 kHz. You will see voltages that are less than 1 volt. What is going in on? The filter is filtering out the high frequencies. Remember, your cutoff frequency is 1.6 kHz. Anything lower than 1.6 kHz is passed through. Anything higher than 1.6 kHz is attenuated/reduced or completely blocked. Ergo, you have low pass filter, low pass, high don't pass.

If instead you hookup the o-scope across the resistor, then you will have high pass filter. Frequencies below 1.6 kHz will be attenuated/reduced below 1 volt. Frequencies above 1.6 kHz will be passed through, ideally, unaltered, 1 volt goes in, 1 volt comes out.

This is the nice thing about voltage divider type RC filter, take your output voltage across capacitor and you have low pass filter, take your output across resistor and you have high pass filter.

3. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
511
Also, remember that your resistor is not perfect. I normally used 5% resistors in my labs. So your prelab calculations will be using 1 kOhm "ideal" resistor. Your lab work will be using the none ideal real life resistor so measure it to find out the true value of the resistor.

For example. My calculated Fc is using ideal 1 kOhm resistor. Your true Fc/experimental Fc will be based on the actual value of the resistor and if you use 5% resistor could be anywhere from 950 Ohm to 1050 Ohm.

4. ### LvW Active Member

Jun 13, 2013
674
100
By describing a circuit with words and using the term "parallel" you always must explicitely state which part is in parallel to WHAT?
The term "parallel" ALWAYS involves TWO elements (better: two two-poles).